Cholesteric liquid crystal polymer (CLCP) film is characterized by the fact that it selectively reflects the incoming light and turns out a narrow band circular polarization to the front viewer. The CLCP film is utilizing “Bragg reflection”, one of the intrinsic properties of cholesterics. In Bragg reflection, only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer. The remaining spectra of the incoming light, however, including the 50% opposite-handed circular polarization and the same-handed out-off Bragg reflection wave band, will pass through the film. Theoretically, the reflective component is narrow band circular polarization while the transmissive one is broadband elliptical polarization.
A broadband reflection can be also realized by means of changing the helical structure of the CLCP film.
The cholesteric broadband polarizer (BBP) was first disclosed, to the knowledge of the applicant, in the article “Cholesteric Color Filter Made From Cholesteric LC Silicones”, published May 15, 1990 (SID 90 Digest. 111). The paper describes experiments concerning the construction of broadband polarizer by combining layers of cholesteric LC-silicones of different reflection wavelength. Five LC-silicon layers were stacked together and the circular polarization was observed from 430 nm to 670 nm. The ellipticity spectra for the combined layers were also calculated from the spectra of each single layer. The good agreement with the observed spectra clearly demonstrates the conservation of circular polarization by transmitting light through cholesteric layers. For that reason it is possible to arrange LC-layers with different Bragg reflection wavelengths to get broadband filters without loss of circular polarization.
The European Patent Application 94200026.6 with the title of “Cholesteric Polarizer and Manufacture Thereof”, published Jul. 20, 1994 and assigned to Philips Electronics, N.V. of Eindhoven, Netherlands (the “Philips reference”) introduces a method to make a single layer CLCP film having broadband reflection and transmission characteristics. The Philips disclosure requires adding a UV dye into CLCP mixture so that the pitch of the CLCP material changes linearly from its maximum value at one film surface to its minimum value at the other film surface. The CLCP material is formed from two polymerizable chiral and nematic monomers, each of which has a different reactivity. During polymerization of the mixture by means of UV exposure, a linear variation in UV light intensity is to be preferentially incorporated into the least reactive monomer to occur at the location of the highest radiation intensity. As a result, at least one concentration gradient of free monomers is formed during polymerization, causing the monomer to diffuse from locations with a low monomer concentration to the location with a high concentration. The monomers of high reactivity diffuse to the locations where the radiation intensity is the highest. As a result, the composition of the material varies in a direction transverse to the surfaces of the film such that a linear variation in the pitch of the molecular helices results in the layer formed by the polymer. The liquid crystal material is distributed linearly across the thickness of the film. This variation in pitch provides the optically active layer with a bandwidth proportional to the variation in the pitch of the molecular helices.
An article “From Selective to Wide-band Light Reflection: a Simple Thermal diffusion in a Glassy Cholesteric Liquid Crystal”, published Dec. 17, 1998, Physical Journal B, France, introduces a method to fabricate a wide-band circular polarizer. The method relates to a spontaneous diffusion of monomers into a polymerizable CLCP film and then following a UV polymerization. The fabrication is carried out by depositing a film of reactive monomers on the surface of a polymerized film of CLCP material. The diffusion of monomers into the CLCP film layer causes a concentration gradient in the layer before diffusion is halted. As a result, the original CLCP material swells slightly causing an increase in pitch of the molecular helices. This provides a concentration gradient which, in turn, results in a “linear variation” in pitch across the film thickness. Polymerization of the layer by UV light exposure halts diffusion providing a broadband polarizer.
U.S. Pat. No. 6,532,049 with the title of “Circularly Polarizing Reflective Material Having Super Broad-band Reflection and Transmission Characteristics and Method of Fabricating and Using Same in Diverse Applications” published Mar. 11, 2003 and assigned to Reveo, Inc. of Elmsford N.Y. (the “Reveo reference”), introduces a method for fabricating a broadband circularly polarizing material. According to the method, a CLCP material is mixed with non-cross linkable liquid crystal material, a photoinitiator and a chiral additive at a temperature, which maintains the mixture in a liquid crystal state. While being heated, the mixture is subjected to UV light radiation for a time and at an intensity sufficient to polymerize the CLCP material or the liquid crystal material or the both. Such polymerization occurs in a non-linear fashion, thereby resulting in a non-linear distribution of the polymer and the liquid crystal material across the film. During polymerization, phase separation takes place. The segregation rate of the liquid crystal material is designed to be greater than the polymerization rate of the CLCP material being polymerized. Thus, the liquid crystal material segregates and diffuses to sites of enlarged pitch in the CLCP material from sites of shrunken pitch in the CLCP material. Consequently, an exponentially distributed pitch is generated from one surface to the other of the CLCP film.
In the above-mentioned prior art, all the broadband polarizers reported are one-dimensional optical system wherein the gradient of the helical pitch variation occurs only in the depth dimension (i.e. transverse to the surface of the substrates) whether is in a linear fashion or in a non-linear fashion. All those prior art products take on a mirror appearance; no wander people often call them “dichroic mirror”. The specula reflection of the traditional BBP film greatly limits itself in many applications because of its strong angular dependence. A separate diffuser layer is usually combined with it in order to enlarge the viewing angle. The other shortcoming of the prior art is that the narrow band circular polarizer and the broadband circular polarizer are not inter-convertible. Once the broadband polarizer is produced, the material will never convert back to the narrow band material.